1. Field of the Invention
This invention relates to a spindle motor for a disc drive used in an information, image or sound instrument such as a magnetic disc, a magneto-optical disc or an optical disc, or for other precision rotation instruments and the like.
2. Related Background Art
A spindle motor for a disc drive according to the prior art is of such structure as shown in FIG. 6 of the accompanying drawings.
In the arrangement of FIG. 6, hub 51 carrying a disc 50 thereon is rotatably supported on a shaft member 54 through two ball bearings 52 and 53. A rotor 56 fixed to the hub 51 is opposed to a stator 57 fixed to a housing 55 supporting the shaft member 54, thus constituting a motor M, and the hub 51 and disc 50 are rotatively driven by the motor M.
Recent advances in magnetic disc drive apparatuses have led to the use of thinner discs and higher recording density. Consequently, a small amplitude and small vibration of the element of non-repetitive runout (vibration which does not synchronize with rotation) of the disc are required of spindle motors used therein.
Accordingly, small vibration of the element of non-repetitive runout is required of the ball bearings incorporated in the spindle motor as shown in FIG. 6. The ball bearings, however, have ball passage vibration and vibration attributable to the errors of the shapes of the parts of the bearings, and even if the accuracy of working is improved, it is difficult to make the vibration of the element of non-repetitive runout less than a predetermined value. Also, when two ball bearings are used, there is the disadvantage that the vibration of the element of non-repetitive runout becomes great due to misalignment attributable to the error of the assembly of the two ball bearings.
On the other hand, there has been studied a spindle motor using, instead of ball bearings, hydrodynamic fluid bearings small in the vibration of the element of non-repetitive runout in both of a radial bearing and a thrust bearing. But the thrust hydrodynamic fluid bearing suffers from the disadvantage that the starting torque is great because the bearing surfaces are in surface contact with each other when the motor is stationary. Also, when the spindle motor is used in its vertically placed state, there is the problem that the starting torque becomes great and a great axial load is applied to the thrust hydrodynamic fluid bearing, so that by the repetition of starting and stoppage, the surface of the thrust bearing is liable to be damaged or worn.
Also, with the downsizing and lighter weight of the apparatus, the use in portable instruments has become widespread. In such applications, small power consumption and small noise are required of the spindle motor.